Bottom Line:
To investigate the role of hypothalamic neuronal circuits in this process, we used a Cre-lox strategy to create mice with specific and progressive degeneration of hypothalamic neurons that express agouti-related protein (Agrp) or proopiomelanocortin (Pomc), neuropeptides that promote positive or negative energy balance, respectively, through their opposing effects on melanocortin receptor signaling.Agrp-ablation female mice exhibit reduced adiposity with normal compensatory hyperphagia, while animals ablated for both Pomc and Agrp neurons exhibit an additive interaction phenotype.These findings provide new insight into the roles of hypothalamic neurons in energy balance regulation, and provide a model for understanding defects in human energy balance associated with neurodegeneration and aging.

Affiliation: Department of Genetics, Stanford University School of Medicine, Stanford, California, USA.

ABSTRACTNormal aging in humans and rodents is accompanied by a progressive increase in adiposity. To investigate the role of hypothalamic neuronal circuits in this process, we used a Cre-lox strategy to create mice with specific and progressive degeneration of hypothalamic neurons that express agouti-related protein (Agrp) or proopiomelanocortin (Pomc), neuropeptides that promote positive or negative energy balance, respectively, through their opposing effects on melanocortin receptor signaling. In previous studies, Pomc mutant mice became obese, but Agrp mutant mice were surprisingly normal, suggesting potential compensation by neuronal circuits or genetic redundancy. Here we find that Pomc-ablation mice develop obesity similar to that described for Pomc knockout mice, but also exhibit defects in compensatory hyperphagia similar to what occurs during normal aging. Agrp-ablation female mice exhibit reduced adiposity with normal compensatory hyperphagia, while animals ablated for both Pomc and Agrp neurons exhibit an additive interaction phenotype. These findings provide new insight into the roles of hypothalamic neurons in energy balance regulation, and provide a model for understanding defects in human energy balance associated with neurodegeneration and aging.

pbio-0030415-g003: Effect of Pomc-Specific Tfam Deficiency on Pomc and Agrp ImmunostainingEach column of panels shows coronal sections from a 7-mo-old control (Tfam flox/+; Tg.PomcCre/+ or Tfam flox/flox; +/+), Pomc-specific Tfam mutant (Tfam flox/flox; Tg.PomcCre/+), and Agrp-specific Tfam mutant (Tfam flox/flox; Tg.AgrpCre/+) on the left, middle, and right, respectively. Each section was stained for α-MSH or Agrp as indicated. The three lower panels show coronal sections stained to reveal cell nuclei (DAPI) and immunostained for GFAP, and indicate that ablation of Pomc or Agrp neurons does not grossly disturb tissue architecture, cell number, or stimulate astrocytosis. The sections shown are representative of three different animals that were examined.IR, immmunoreactivity.

Mentions:
We examined the pattern and extent of immmunohistochemical staining for α–melanocyte-stimulating hormone (α-MSH) and Agrp in Tfam mutant and control mice at multiple time points. At 2 and 4 mo of age, α-MSH and Agrp neurons in Tfam mutant mice appeared no different from control mice (Figure 2). However, at 7 mo of age, we found that in Pomc-specific Tfam mutant mice, immunohistochemical staining for α-MSH was dramatically reduced compared to control animals, but immunohistochemical staining for Agrp was unaffected (Figure 3). Similarly, in Agrp-specific Tfam mutant mice, immunohistochemical staining for Agrp was dramatically reduced compared to control animals, but immunohistochemical staining for α-MSH was unaffected (Figure 3). In both cases, there was no detectable gliosis or effect on brain architecture, indicating that the ablation was highly specific.

pbio-0030415-g003: Effect of Pomc-Specific Tfam Deficiency on Pomc and Agrp ImmunostainingEach column of panels shows coronal sections from a 7-mo-old control (Tfam flox/+; Tg.PomcCre/+ or Tfam flox/flox; +/+), Pomc-specific Tfam mutant (Tfam flox/flox; Tg.PomcCre/+), and Agrp-specific Tfam mutant (Tfam flox/flox; Tg.AgrpCre/+) on the left, middle, and right, respectively. Each section was stained for α-MSH or Agrp as indicated. The three lower panels show coronal sections stained to reveal cell nuclei (DAPI) and immunostained for GFAP, and indicate that ablation of Pomc or Agrp neurons does not grossly disturb tissue architecture, cell number, or stimulate astrocytosis. The sections shown are representative of three different animals that were examined.IR, immmunoreactivity.

Mentions:
We examined the pattern and extent of immmunohistochemical staining for α–melanocyte-stimulating hormone (α-MSH) and Agrp in Tfam mutant and control mice at multiple time points. At 2 and 4 mo of age, α-MSH and Agrp neurons in Tfam mutant mice appeared no different from control mice (Figure 2). However, at 7 mo of age, we found that in Pomc-specific Tfam mutant mice, immunohistochemical staining for α-MSH was dramatically reduced compared to control animals, but immunohistochemical staining for Agrp was unaffected (Figure 3). Similarly, in Agrp-specific Tfam mutant mice, immunohistochemical staining for Agrp was dramatically reduced compared to control animals, but immunohistochemical staining for α-MSH was unaffected (Figure 3). In both cases, there was no detectable gliosis or effect on brain architecture, indicating that the ablation was highly specific.

Bottom Line:
To investigate the role of hypothalamic neuronal circuits in this process, we used a Cre-lox strategy to create mice with specific and progressive degeneration of hypothalamic neurons that express agouti-related protein (Agrp) or proopiomelanocortin (Pomc), neuropeptides that promote positive or negative energy balance, respectively, through their opposing effects on melanocortin receptor signaling.Agrp-ablation female mice exhibit reduced adiposity with normal compensatory hyperphagia, while animals ablated for both Pomc and Agrp neurons exhibit an additive interaction phenotype.These findings provide new insight into the roles of hypothalamic neurons in energy balance regulation, and provide a model for understanding defects in human energy balance associated with neurodegeneration and aging.

Affiliation:
Department of Genetics, Stanford University School of Medicine, Stanford, California, USA.

ABSTRACTNormal aging in humans and rodents is accompanied by a progressive increase in adiposity. To investigate the role of hypothalamic neuronal circuits in this process, we used a Cre-lox strategy to create mice with specific and progressive degeneration of hypothalamic neurons that express agouti-related protein (Agrp) or proopiomelanocortin (Pomc), neuropeptides that promote positive or negative energy balance, respectively, through their opposing effects on melanocortin receptor signaling. In previous studies, Pomc mutant mice became obese, but Agrp mutant mice were surprisingly normal, suggesting potential compensation by neuronal circuits or genetic redundancy. Here we find that Pomc-ablation mice develop obesity similar to that described for Pomc knockout mice, but also exhibit defects in compensatory hyperphagia similar to what occurs during normal aging. Agrp-ablation female mice exhibit reduced adiposity with normal compensatory hyperphagia, while animals ablated for both Pomc and Agrp neurons exhibit an additive interaction phenotype. These findings provide new insight into the roles of hypothalamic neurons in energy balance regulation, and provide a model for understanding defects in human energy balance associated with neurodegeneration and aging.